CHAPTER 4. Design of Innovative Creep Testing Facility under Flowing. Sodium

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1 CHAPTER 4 Design of Innovative Creep Testing Facility under Flowing Sodium 4.1 Introduction In this chapter, design of an innovative creep testing system comprising of a lever arm type creep machine with auto adjustable fulcrum to maintain lever horizontality and doublebellow creep specimen environmental chamber with unique safety features for carrying out creep tests in flowing sodium is presented. A new concept has also been incorporated in the testing system to counter balance the reduction in load on the specimen resulting from the stiffness of bellows, as the specimen elongates during creep. 4.2 Design requirements of an environmental chamber for conducting creep tests in flowing sodium The following features of the environmental chamber are required to carry out creep test in flowing liquid sodium i. Arrangement to maintain constant flow rate of liquid sodium across the specimen. ii. Load should not be induced on the bellows by the flowing sodium and controlled amount of flowing sodium should enter the chamber. iii. Use of less number of bellows to decrease the stiffness of the bellow system so that the creep specimen experiences the desired preset stress. iv. Provision for secondary bellow to contain the leaked liquid sodium from the primary bellow form safety point of view. 65

2 v. Provision for automatic detection of leaked liquid sodium. vi. Maintaining inert gas atmosphere in the spaces between the bellows to avoid direct contact of the leaked liquid sodium with air. 4.3 Design of an environmental chamber for conducting creep test in flowing liquid sodium. A schematic of the specially designed creep testing environmental chamber with the above mentioned features is shown in Fig A unique co-axial bellow system has been developed to mechanically isolate the creep specimen from the testing chamber as well as to provide secondary protection to the leaked liquid sodium in the event of rupturing of the primary bellow using minimum number of bellows. In this design, the bellow will not carry the load of the flowing sodium. During creep test, the primary bellow will contract and the secondary bellow will expand to accommodate the specimen elongation. The bellows have been welded with the environmental chamber without using any O ring from safety point of view. The annular space available between the two bellows has been filled with argon gas with slightly positive pressure than the atmosphere. Spark plug is installed in the spaces between the two bellows to detect sodium leak. Specimen chamber (Fig. 4.1) in which the test specimen is located in the creep testing environmental chamber is of double conical shaped to provide an annular space along the specimen for maintaining constant flow rate of the liquid sodium across it. Liquid sodium under constant pressure enters the specimen chamber through the inlet and leaves through the outlet. Top and bottom pull rods of the creep testing environmental chamber are connected with the load train of the creep testing machine. The creep testing environmental chamber including the bellows is made of 316L(N) SS. These have been assembled and fabricated by welding 66

3 with 100 % quality control. The primary bellow is of seamless type design and the secondary bellow is of welded type design. 4.4 Design and performance of bellows Bellows are employed in the creep testing chamber to isolate the creep specimen and act as a secondary containment for receiving of any leaked sodium through primary bellows. Two bellows namely primary and secondary are arranged in a co-axial manner. The latter is made of seamless type and former is of welded type bellow of grade 316L SS. Basically it is a single bellow system and is suited for linear elongation and compression with no lateral movements and as angular rotation. Primary bellow working under sodium atmosphere where as secondary bellow is surrounded by argon gas and sodium might be filled at the time of failure of the primary bellow. General arrangement of primary bellow is shown in Fig One end of the bellow (bottom) is welded with the top pull rod and the other end is welded with stationery part of the creep chamber as shown in Fig A top pull rod is welded to the primary bellow and the outer surface of this bellow faces the flowing sodium. Top end of bellow is welded with top flange of the sodium vessel as shown in Fig Normally, as per Expansion Joints Manufacturers Association (EJMA) standard [126], bellow is defined as any device which is used to absorb dimensional changes which are caused by thermal expansion or contraction of a pipeline, duct or vessel. Primary bellow is used here to allow the creep specimen to elongate along with top pull rod and is isolated from the stationary part of the chamber (specimen chamber, bottom pull, inlet/outlet of sodium pipes etc.). This bellow is designed in such a way that to minimise the stiffness. Like a conventional bellow, cycles are one of the important parameter for design of bellow. Stiffness of the bellow is one of the major design criteria to avoid major load shared from the tare load of the specimen. Minimum stiffness (0.13 Kgmm -1 ) at testing temperature is adopted in the chamber; hence the load induced from the mini lever system to the load train of the specimen is considerably 67

4 reduced. Micro plasma tungsten inert gas (TIG) welding process was used for joining the bellow with pull rod and top flange of the chamber. Specifications of primary bellow are given in Table 4.1. Secondary bellow is arranged in a co-axial manner to primary bellow and its purpose is to collect the leaked sodium from the main chamber when leak occurs in case of failure of primary bellow. It is a welded type of single bellow system and its design parameters are also as per EJMA standard. During the operation, this bellow tends to expand. A general arrangement of secondary bellow is as shown in Fig Integration of bellow with chamber is typically with primary bellow system. Top end of the bellow is welded with top pull rod of the chamber and bottom is welded with stationary part of the chamber as shown in Fig Unlike the conventional cycle experienced by the bellow, in this system there is no cycle function during the creep test. Instead, the primary bellow is slowly loaded into compression stage during creep test and halts after specimen failure (Fig. 4.4). Table 4.1 Specifications of Seamless bellow (Primary) Sl. No. Nomenclatures Specifications 01 Type Metallic Bellows- Seamless 02 Materials of construction 316L (SS) 03 Design pressure a. External b. Internal 6 Kg/Sq. cm (g) 1 Kg/Sq. cm (g) 04 Test pressure 20 Kg/Sq. Cm(g) 68

5 Sl. No. Nomenclatures Specifications 05 Design Temperature 650 C at ambient temperature 06 No. of walls 2 Nos. 07 Bellows dimensions a. Overall length b. Active length c. Outer diameter d. Inner diameter 275 mm 235 mm 31.5 mm 18.5 mm 08 Axial movement 40 mm 09 Axial spring rate 0.13 Kg/mm 10 No. Of cycles End type Flanged 69

6 Fig.4.1 A schematic of designed environmental chamber for carrying out creep test in flowing sodium with details of specimen chamber. 70

7 Fig. 4.2 Seamless Bellows arrangements (Primary) 71

8 Fig. 4.3 Welded bellows (Secondary) 72

9 Fig. 4.4 Deforming of primary and secondary bellows during creep tests at high temperature 73

10 4.5 Design of creep testing machine for conducting creep test in flowing liquid sodium incorporating the environmental chamber A schematic of the creep testing machine incorporating the environmental chamber is shown in Fig The difference between the mechanisms adopted to maintain the lever horizontality in the conventional and the specially designed creep testing machine are schematically explained in Fig In the case of conventional creep testing machine, the load train along with specimen moves downwards by drawhead mechanism to maintain the lever horizontality. In the specially designed creep machine, the fulcrum has been moved vertically by a drawhead mechanism to maintain the lever horizontality without requiring any movement of load train with environmental chamber welded with it. A sensor has been provided to detect the deviation of lever horizontality and to activate the draw head mechanism to move the fulcrum upwards. In the event of specimen failure, an arrest mechanism has been provided to avoid rupturing of the bellows due to excess displacement and also provision has been made to switch off the power to creep machine. Two bellows were employed in creep chamber to mechanically isolate the creep specimen from the chamber. The load train moves up as the specimen elongates during creep. This causes compression of the primary bellow and extension of the secondary bellow from their free length positions. The stress applied to the creep specimen is being shared by the bellows due to its stiffness and hence the stress on the specimen is being progressively reduced on creep exposure. To counter balance the load shared by the bellows, a mini lever mechanism has been attached with the load train as shown in Figs. 4.5 and 4.7. Load produced by the mini lever is directly transmitted to the load train. A dead-weight is connected to the mini 74

11 lever just above the load train through a rigid rod. The initial load provided by the mini lever to the load train will be that produced by the effort applied on it minus the dead-weight. This load is being taken into account while loading the main lever to apply the specific load on the creep specimen. As the load train moves up along with the fulcrum due to specimen elongation, the mini lever moves upwards also as shown sequentially in Figs This will bring the dead-weight close to the fulcrum of the mini lever as the dead-weight moves on an arc as shown in Fig This will cause the effective load applied by the mini lever to the load train to increase since the dead-weight is not directly above the load train. The mini lever size and quantity of the dead-weight along with height of the rigid rod are designed in such a way that their movement just counter balances the load shared by the bellows as the load train moves due to the elongation of the creep specimen. This provides constant load to the specimen during creep test. Unlike in the conventional creep testing machine, no furnace is provided to maintain constant temperature across the specimen in the presently designed machine. Preheated liquid sodium having temperature equal to the creep test temperature is circulated around the specimen. Surface heater has been wrapped over the environmental chamber. Temperature of the flowing liquid sodium is maintained within ±2 K of testing temperature by controlled supply of power to the surface heater through PID temperature controller and thyristor power supply unit. The temperature of the creep specimen is sensed by a K-type thermocouple embedded on the specimen chamber. The elongation of the specimen is measured by the relative displacement of the load train with respect to the creep machine frame by a digital dial indicator attached with the frame of the creep machine. A load cell has been incorporated in the load train. The elongation and 75

12 temperature of the specimen and load in the load train were logged in a data logging system throughout the creep test and displayed on a computer screen. Creep test can be carried out according to ASTM E standard procedures and requirements. [127]. 76

13 Fig.4.5 A schematic of the designed creep testing machine 77

14 Fig.4.6 A schematic, illustrating beam leveling mechanism in (a) conventional creep testing machine and (b) in the designed machine. 78

15 Fig.4.7 A sketch showing details of the draw head mechanism to adjust the position of fulcrum to maintain lever horizontality. 79

16 Fig. 4.8 Mechanism for lever horizontality Lever position at the beginning of creep test 80

17 Fig. 4.9 Mechanism for lever horizontality Lever position as the specimen elongate during creep test 81

18 4.10 Mechanism for lever horizontality Lever horizontality is maintained by moving fulcrum up without disturbing the load train 82

19 4.6 Mechanism to counter balance the stiffness of bellow The load train moves up as the specimen elongates during creep. This causes compression of the primary bellow and extension of the secondary bellow from their free length positions. The stress applied to the creep specimen gets shared by the bellows due to their stiffness and hence the stress on the specimen is being progressively reduced on creep exposure. To counter balance the load shared by the bellows, a mini lever mechanism has been attached with the load train, as shown in Figs. 4.7 and 4.9. The functioning of the mini lever for applying additional load that is being shared by the bellows to the creep specimen for maintaining constant load on it during creep exposure is described in Fig The load produced by the mini lever is directly transmitted to the load train. A dead-weight is connected to the mini lever just above the load train through a rigid rod. The initial load provided by the mini lever to the load train will be that produced by the effort applied on it minus the dead-weight. This load is taken into account while loading the main lever to apply the specific load on the creep specimen. As the load train moves up along with the fulcrum due to specimen elongation, the mini lever also moves upwards. This will bring the deadweight closure to the fulcrum of the mini lever as the dead-weight moves on an arc, as shown in Fig This will cause the effective load applied by the mini lever to the load train to increase since the dead-weight is not directly above the load train. The mini lever size and the weight of the dead-weight along with height of the rigid rod are designed in such a way that their movement just counter balances the load shared by the bellows as the load train moves due to the elongation of the creep specimen. This provides constant load to the specimen during the creep test. 4.7 Safety features A secondary containment bellow around the primary bellow with argon gas filled in between the annular spaces of the bellows has been provided for safety purpose. A spark plug liquid 83

20 Fig.4.11 Mini lever system for counterweight to maintain tare load of the specimen. 84

21 Fig Mini lever system to maintain tare load of the specimen during the creep tests 85

22 metal detector and pressure gauge with safety relief valve are attached with the secondary bellow to detect leaked liquid sodium from the primary bellow. 4.8 Procedure for conducting creep test in flowing sodium i. Environmental chamber along with the load train is integrated with the creep testing machine. ii. Pipes circulating liquid sodium are welded with the inlet and outlet of the specimen chamber. iii. Annular space between the primary and secondary bellows of the environmental chamber is filled with argon gas by evacuating the air by vacuum pump up to 10-1 bar and then purged with argon gas. The process of evacuating and purging are carried out three times to ensure almost complete removal of residual air in the space. A positive argon gas pressure is maintained inside the spaces between the bellows by closing the valves. iv. Air in the specimen chamber of the environmental chamber is evacuated by vacuum pump up to pressure of 10-1 bar. Argon gas is allowed to purge in the specimen chamber. The processes are repeated three times to remove the residual air almost removed. A positive argon gas pressure is maintained inside the specimen chamber. v. Liquid sodium is allowed to flow inside the specimen chamber by opening the valves. Constant pressure of the liquid sodium is maintained for constant flow rate of liquid sodium across the specimen. vi. Temperature of the flowing sodium in the specimen chamber is maintained within ±2 K of the test temperature by surface heater and PID controller. vii. Creep test is carried out as per ASTM E [128]. 86

23 viii. Temperature and elongation of the test specimen and load in the load train are logged in a data logger throughout the creep test. ix. On completion of the test, valves are closed to stop liquid sodium flow. x. Drainage of liquid metal is then carried out from the chamber to the dump tank. 4.9 Data logging system The thermocouples, load cell and digital dial indicators are connected to a data logging system, the block diagram of the data logging system is shown in Fig The system consists of a PC and a data scanner for interlinking of all data from the sodium creep testing machine. The mode of handshake is through RS232 cables. The data logger shows the online values of the elongation, temperature and stress values in term of numbers as well as graphs. After rupture, creep data can be obtained from the system and can be later analyzed Fabrication of environmental chamber Detailed fabrication drawing of the environmental chamber is shown in Fig All components except the pull rods have been machined from SS 316L(N) SS. The pull rod is fabricated by Inconel 617 material. A primary Bellow (Part No. 13) of SS 316 L material is welded to top pull rod (Part No. 1) by approved welding procedures. Inconel 186 is used as filler wires. This primary bellow assembly was tested with 100 % LPI and Helium leak. Bottom pull rod (Part No. 10), bottom cup (Part No. 9) and inlet pipes ( Part Nos. 6 & 15) are assembled as per the drawing. Outlet pipes with chamber pipes (Part No. 4) and primary bellows assembly is assembled and welded. Welding quality was inspected with 100 % liquid penetrating examination. Secondary bellows (welded type, Part No. 2) with its top cover (Part No. 11) and cup (Part No. 14) to carry out secondary bellows (bottom) is welded to top pull rod assembly. On the bottom cup flange of secondary bellows on one side, sparks plug (Part 87

24 No. 5) is integrated and connected with argon gas circuit. Creep specimen (Part No. 8) is connected with bottom and top pull rods by threading and subsequently conical shaped specimen cup (Part No. 8) is welded to bottom cup and pipes of chamber as per drawing. Assembly is tested for liquid penetrant examination, radiography testing for butt joints, pneumatic testing and finally helium leak testing as per ASME code [127]. Details of different parts of the environmental chamber are given in Figs Fig Establishment of data logging system in creep tests under flowing sodium 88

25 SODIUM OUTLET SODIUM INLET Fig Fabrication system of sodium creep testing chamber All dimensions are in "mm 89

26 DETAIL OF ITEM NO.1 DETAIL OF ITEM NO.11 DETAIL OF ITEM NO.12 Fig Details for Item No. 01, 11 & All dimensions are in mm

27 DETAIL OF ITEM NO.10 DETAIL OF ITEM NO.14 All dimensions are in mm Fig Details for Item No. 10 & 14 91

28 DETAIL OF ITEM NO.4 DETAIL OF ITEM NO.7 DETAIL OF ITEM NO.8 All dimensions are in mm Fig.4.17 Details for Item No. 4, 7 & 8 92

29 4.11 Validation of the creep testing machine designed Creep tests were carried out on the 316L(N) SS at 873 K over a stress range of MPa in air and in flowing sodium environments and actual performance of the machine and the test parameters are shown in Fig The velocity of the sodium was maintained at around 2.5 m/sec. Typical creep curves of the steel at 225 MPa and 873 K obtained by performing creep tests in flowing sodium and air environments are compared in Fig The creep deformation of the steel in both the environments is well characterized by a small instantaneous strain on loading, a transient primary stage, a secondary stage followed by a tertiary stage of creep deformation. The creep rupture life of 316L(N) SS at 873 K was longer in flowing sodium environment than that in air, the extent of which is more at lower applied stresses (Fig. 4.20). Inert atmosphere provided by flowing sodium has been considered to improve creep rupture life of 316L(N) SS. Fig.4.18 Maintaining of test parameters (temperature and load) at 225 MPa creep test under flowing sodium using newly designed creep testing machine 93

30 In flowing sodium In air - Present design machine In air - conventional creep mechine 225 MPa Strain Time, hour 873K 316L (N) Fig.4.19 Maintaining of test parameters (temperature and load) at 225 MPa creep test under flowing sodium using newly designed creep testing machine. Fig.4.20 Creep curves of 316L(N) austenitic stainless steel at 873 K over a stress range of MPa conducted by the innovated creep testing machine in liquid sodium at the flow rate of 450 litre/hour. 94

31 4.12 Conclusions A creep testing machine to carry out creep test in flowing liquid sodium has been designed, fabricated, commissioned and validated. Innovative features of the designed machine are (i) design of an environmental chamber in such a way that the bellows required were mechanically isolated from the creep specimen and were not subjected to the load of the liquid sodium. (ii) use of minimum number (2 nos.) of bellows with the requirement of secondary confinement bellow over the primary bellow from the safety point of view, (iii) maintenance of creep machine lever horizontality by automatically adjusting the position of the fulcrum without disturbing the load train to ensure constant stress on the specimen during the creep test, and (iv) auto adjustment of load in the load train to counter balance the effect of bellow stiffness as the specimen elongates for maintaining constant load on creep specimen. 95